i've been reading, once again, the book Rules of Play by Katie Salen and Eric Zimmerman. It's a good book, recommended to anyone aspiring to be a game designer.
the second chapter is called The Design Process, which is iterative (the process, not the chapter). according to the authors, and i agree: "designers learn best trough the process of design, by directly experiencing the things they make".
the sentence ultimately means that the single most important task a game designer has during the process of creating a game is PLAYING the game he is creating. that lead us to a question a ask anyone that comes for a game designer job interview: How much do you play?
a decent game designer can't play less then four hours a day. why? during the process of making a game, the game designer will be playing the game over and over again for hours troughout months of development, most of this time the game will be an unbalanced, ugly and full of bugs piece of crap.
the funny thing is that, at least in my experience, three quarters of the people postulating a game design position can't even say the last game they played.
so, if you want to be a game designer start playing. play the same game over and over again. pay attention to every single detail. know what is right and what is wrong about the game. learn what makes it a good game and what makes it a sucessful game. then, when you go to an interview you can tell everything about it and make sure the person who is interviewing you gets you are simply passionate about what you are talking about...